Combined gas and steam turbine installations are installations in which the waste heat created in the gas turbine part is thermally coupled, in the form of exhaust gases, to a waste-heat boiler for operation of a steam turbine installation. The exhaust-gas temperature of the gas turbine is normally kept as constant as possible over a wide power range, although this restricts rapid load changes on the gas turbine installation. The load change capability is essentially limited by the dynamics of the exhaust-gas temperature regulation, and therefore the capability to vary the air mass flow of the gas turbine. The steam turbine part of the installation follows the power changes of the gas turbine essentially with a considerably more inert time response. In consequence, in the case of combined gas and steam turbine installations in network operation, it is normal to cope with any power changes required by the network, in particular for network frequency stabilization purposes, solely by the respective gas turbine part of the installation, not least because the steam turbine part cannot contribute anything to this in the first seconds. It is therefore necessary for the total reserve power for power regulation in network operation to be provided by the gas turbine; a situation which in the end results in the block power which is maintained in the steady state of the gas turbine installation to cover the reserve power being reduced by a corresponding power component in order to make it possible to compensate for the delayed contribution of the steam turbine in seconds, not least because the network operator is obliged to guarantee a specific alternating-current frequency, for example at 50 Hz in Europe, which is stable over time and with respect to the demanded electrical power. The frequency stability in the network is ensured by dynamic load/power matching, for which it is necessary to ensure that considerable reserve powers are available within seconds.
EP 1 301 690 B1 discloses a method relating to this for primary regulation of a combined gas and gas turbine installation, in which the steam turbine is operated, in order to quickly provide reserve power, such that at least one pressure stage of the steam part is operated with a restricted valve position, thus forming a frequency support power reserve which is used for frequency support in the event of an underfrequency on the network side, by reducing the restriction of the actuating valve in accordance with the extent to which the frequency is undershot. The time period of reduced restriction is in this case designed on the basis of a restricted-time fading signal.
The restricting of the actuating valves results in a ram-air pressure being built up in front of the respective pressure stage in the steam turbine part, which can be released in the form of stored reserve power as required by opening the actuating valves, with the magnitude of the reserve power depending on the extent of the restriction. If the restriction is removed, then the ram-air pressure builds up slowly, as a result of which the stored reserve power can be additionally emitted by the steam turbine, in the form of so-called primary regulation power. Operation of the steam turbine in this way on the one hand makes it possible to provide a higher steady-state block power, which can be provided in particular by the gas turbine installation, while on the other hand the steam turbine can provide its reserve power component virtually without any time delay, that is to say within a few seconds, which in the end leads to a total delay-free and greater primary regulation power of the combined gas and steam turbine installation.